In the European context, a railway train for the transport of goods typically comprises a locomotive which hauls a plurality of wagons. The maximum length of a train of this type is specified by the European standards authority UIC (Union Internationale des Chemins de Fer) and is based on the functional characteristics of the braking system, which are also governed by this authority.
Some fundamentals of railway braking technology in the prior art will now be recapitulated in order to facilitate the understanding of the present invention.
A locomotive is typically equipped with a device, namely a brake valve or cock, which can be operated by the driver to regulate the pneumatic pressure in the brake pipe which extends from the locomotive throughout the whole train to the last wagon.
The brake pipe serves to supply and control all the local braking apparatuses in each component vehicle of the train. According to the UIC standards for this brake pipe, as shown in the graph in FIG. 1, which correlates the value of the braking pressure pCF at the brake cylinders with the pressure pCG in the brake pipe, the pressure in the pipe may have nominal operating values in the range from 3.5 bar to 5 bar: the value of 3.5 bar corresponds to a condition of full braking of the train, while 5 bar corresponds to a condition of zero braking of the train, and intermediate pressure values, rising from 3.5 to 5 bar, correspond to train braking forces decreasing proportionately from full braking to zero braking.
On the initial application of the brakes, the braking pressure reduction is normally applied by means of a first negative pressure “step” of 0.5 bar, with a rather large time increment Δp/Δt, compared with the subsequent variation of said pressure. This pressure step is propagated like a sound wave within the brake pipe, at a speed close to the speed of sound in air. During the propagation in the pipe, the amplitude of said negative pressure step decreases because of frictional interference between the air and the walls and bends of the pipe.
A further reduction of the pressure in the pipe from 3.5 bar to 0 bar corresponds to the complete evacuation of the pipe, which takes place in emergency braking: this condition favours the quickest possible evacuation of the pipe, corresponding to the quickest possible application of the brakes.
Each vehicle of a railway train is equipped with a braking device, known as a distributor valve, designed to execute certain functions that are described below.
FIG. 2 shows a pneumatic diagram of the braking system of a generic vehicle in a train: a distributor valve DV, connected to the brake pipe BP and to a control reservoir CR, sends a braking pressure to brake cylinders BC, transferring air which is stored at 5 bar in an auxiliary reservoir AR, according to the diagram of FIG. 1.
The distributor valve DV has the following three main functions:                applying and releasing the braking force at the vehicle in accordance with the pressure variations in the brake pipe BP, according to the transfer function of FIG. 1;        re-supplying air to the auxiliary reservoir AR: when the pressure in this reservoir is below the pressure in the brake pipe BP after an application of the brakes, the distributor valve DV proceeds to transfer air from the pipe BP to the auxiliary reservoir AR until their pressures become equal; therefore, when the lead locomotive of the train commands the full release of the train brakes by means of the distributor valve DV, the latter restores the pressure in the pipe BP to 5 bar, and consequently the distributor valves of the wagons increase the pressure in the respective auxiliary reservoirs AR to 5 bar, thereby equalizing the pressure in the pipe BP; the time taken for the full release of the train brakes is thus substantially equal to the time taken for the complete filling of all the auxiliary reservoirs AR to 5 bar;        regenerating and propagating the first pressure “step” of 0.5 bar: the distributor valve DV can detect the presence in the pipe BP of the characteristic gradient Δp/Δt of the first braking “step”, even if its initial amplitude of 0.5 bar is partially attenuated as a result of the propagation in the brake pipe BP; when the presence of this gradient Δp/Δt has been detected, the distributor valve DV instantaneously draws a predetermined quantity of air from the pipe BP, such that the value of the pressure “step” is immediately restored to the nominal 0.5 bar; thus the pressure “step” is “regenerated” and propagated towards the distributor valve of the next vehicle in the train; this sequence of events takes place rapidly, thereby preventing any overall delay in the propagation of the aforesaid pressure step along the brake pipe BP.        
The train's locomotive contains a driver's brake valve (DBV), which is an electro-pneumatic device, a schematic diagram of which is shown in FIG. 3. This device typically comprises a relay valve RV, whose reference pressure in the control chamber, henceforth termed the “set point”, is controlled by a solenoid charging valve LV and by a solenoid discharge valve DV, controlled by an electronic unit ECU which interfaces with the driver's brake handle BM. If the partial or total release of the train's brakes is requested, the ECU provides a set point at a pressure of more than 3.5 bar, according to the driver's request, or equal to 5 bar if a total release is requested. The relay valve RV then starts to supply air to the brake pipe BP, drawing air from a main reservoir MR supplied by a compressor (not shown) at pressures of more than 5 bar, typically within the range from 8 bar to 10 bar, for example, interrupting the supply of air to the pipe BP only when the pressure therein has nominally equalled the set point value minus the tolerances of the relay valve.
As described above, during this step of filling the brake pipe BP, the air supplied to this pipe from the main reservoir MR through the relay valve RV contributes to the additional filling of the auxiliary reservoir AR on board each vehicle of the train.
If the partial or total application of the train's brakes is requested, the control unit ECU provides a set point pressure of less than 5 bar, according to the driver's braking request, decreasing to 3.5 bar in the case of maximum braking. After this action, the relay valve RV starts to “evacuate” the brake pipe BP by discharging air to the atmosphere, and interrupts the evacuation of this pipe BP only when the pressure therein has nominally equalled the set point value minus the tolerances of the relay valve.
When an emergency braking request is received, the pressure set point in the pipe BP is brought to 0 bar and the relay valve RV causes the complete evacuation of the pipe BP. At the same time, a high-flow emergency valve, connected between the relay valve RV and the pipe BP (and not shown in FIG. 3) is immediately opened in order to safely reduce the pressure in the pipe BP to zero, accelerating the process of its evacuation.
The length of the train evidently affects, in a substantially proportional manner, the filling and evacuation times of the pipe BP, since an increase in the length of the train is accompanied by an increase in the quantity of air to be introduced into or removed from the pipe BP through the relay valve RV and the emergency valve, and an increase in the retarding effect due to the friction of the air against the walls of the pipe BP.
In particular, because of this friction, during the dynamic phase of the phenomena of application or release of the brakes, the pressure in the pipe BP along the train is non-uniform for relatively long periods.
The graph in FIG. 4 shows the variation in time of the pressures in the brake pipe BP of a 1,200 m long train composed of 50 vehicles, as a result of the application of emergency braking, these pressures being measured at each vehicle: the lower curve in FIG. 4 corresponds to the variation of pressure at the vehicle next to the locomotive, while the upper curve corresponds to the variation of the pressure at the last vehicle. It can be seen that the pressure in the first vehicle reaches the value of 3.5 bar, corresponding to the maximum braking value, in about 3 s, while the pressure in the last vehicle reaches the value of 3.5 bar in about 37 s.
Therefore, as shown by the graphs in FIG. 5, the braking pressure at the brake cylinders of the last vehicle in the train, shown here by the curve farthest to the right, reaches the maximum value with more than 30 s of delay relative to the first vehicle, indicated by the curve farthest to the left.
Similarly, when the brakes are released, the pressure in the pipe BP at the head of the train immediately matches the set point value, while the pressure at the rear of the train slowly increases in value until the whole quantity of air required to rebalance the pressures between the head and the rear and to fill all the auxiliary reservoirs (AR) has travelled from the main reservoir MR to the pipe BP through the relay valve RV. Increasing the flow rate of the relay valve RV does not resolve these problems, since the limits of the flow rate of the valve/brake pipe system are, for these lengths, determined virtually exclusively by the fluid dynamic friction of the air in the pipe BP.
The phenomena of transient non-uniformity of the pressure along the pipe BP described above have a direct effect on the dynamic behaviour of the moving train, creating longitudinal compressive forces along the train, such that conditions favourable to derailment are created. The UIC standards allow for these phenomena and, on the basis of European parameters regarding the maximum weight limits per axle, and the forces that can be achieved by the braking systems to achieve the stopping distances stipulated in European regulations, specifies a maximum safe length of 750 m for travelling trains.
A length of 750 m for goods trains now constitutes a limit which reduces the efficiency of the goods traffic: European railway operators need to be able to operate with longer trains of up to 1,500 m in length, that is to say twice the present safe limit set by the UIC standards.
To this end, it is necessary to limit the non-uniformity and delays in pressure equalization along the pipe BP during the transient in the braking phase, thereby reducing the longitudinal compressive stresses and the associated risks of derailment to safe levels. It should also be borne in mind that trains of this length, when formed with particularly heavy vehicles, may require tractive forces much higher than those that can be developed by a modern European locomotive.
It is known that a solution exists in the UIC area for increasing the traction capacity: it consists in coupling the lead locomotive to at least a second locomotive, controlled by the lead locomotive via an interconnecting cable. However, this solution has limitations, due to the fact that the maximum tractive force that the two locomotives can apply to the train corresponds to the ultimate stress of the coupling between the second locomotive and the rest of the train, and the fact that the problem of improving pressure equalization in the pipe BP during braking transients remains unsolved.
In the American railway area, regulated by the provisions of the AAR (Association of American Railroads) there is a known solution which tackles the problem more effectively; this solution consists in the use of a method called “Distributed Power”, schematically illustrated in FIG. 6, where two or more locomotives are used in a railway train TC, namely a master locomotive ML at the head of the train TC and one or more slave locomotives SL, provided within the train, between the wagons W of the train. This train is associated with a control system known by the name Locotrol (registered trademark), the main characteristics of which will now be briefly described to improve the subsequent understanding of the present invention.
As shown schematically in FIG. 7, this control system comprises a master system MS, located on the master locomotive ML, and one or more slave systems SS, each located on a slave locomotive SL. The master system MS communicates with the slave system(s) SS via a dedicated radio channel, operating on frequencies of about 450 MHz, supported by a predefined communications protocol. The master system MS detects in real time the actions of the driver on the control devices for traction TRC and braking BRC, and transmits corresponding data to the slave system(s) SS which have the function of locally “repeating” the commands imparted by the driver to the traction and braking apparatuses of the slave locomotives SL.
In the traction phase, having a plurality of locomotives distributed along the train, with synchronized traction commands, enables better use to be made of the available tractive force, since the peak tensile stress is not concentrated on the coupling downstream of the lead locomotive(s), as in the European solutions with twin lead locomotives, but is distributed among all the couplings downstream of the various locomotives distributed along the train. The result is equivalent to having the train divided into two or more sub-trains, individually hauled by the distributed locomotives.
During the application of the brakes, having a plurality of locomotives distributed along the train, with synchronized braking commands, means having evacuation points of the brake pipe BP distributed along the train. The result is equivalent to having the train divided into sub-trains defined by the position of the slave locomotive(s), where each segment of the brake pipe BP for each sub-train is individually evacuated at its ends. This is manifested in a substantial reduction in the non-uniformity of pressure in the pipe BP, and in the peaks of longitudinal stress along the train, during the braking transient. Furthermore, braking takes place in much shorter time intervals, corresponding to the reduction in the evacuation time of the pipe BP.
Similarly, during the release of the brakes, having a plurality of locomotives distributed along the train, with synchronized brake release commands, corresponds to having filling points of the pipe BP distributed along the train. In this case also, the result is equivalent to having the train divided into sub-trains defined by the position of the slave locomotives, where each segment of the pipe BP for each sub-train is individually filled at its ends, with a substantial reduction in brake release times.
The system reacts to a loss of radio communication between the master system MS and the slave systems SS according to a procedure known as “Comm Loss Idle Down”, a description of which can easily be found in the railway literature, also available on the internet. According to this procedure, the slave systems SS start to progressively reduce any tractive effort present at the slave locomotives SL to a zero value, and simultaneously isolate the control of the pipe BP on the slave locomotives SL, leaving the task of evacuating the brake pipe BP, if a braking request is received from the driver, to the master locomotive ML alone. However, this “degraded” mode is not considered to be a hazardous situation in the AAR area, since the requirements for braking systems and stopping distances according to the AAR are much lower than the equivalent UIC requirements, and therefore the delays in evacuation and the non-uniformity of pressure in the brake pipe BP during braking transients are acceptable.
Additionally, the compression coupling between vehicles using the AAR couplings provides greater stability in the train and greater tolerance to longitudinal stresses, compared with the compressive couplings using buffers according to the UIC, especially when a train has to negotiate a bend. In general terms, the braking of an AAR railway train having two or three times the length of a UIC train, performed by evacuating the pipe BP from the master locomotive ML only, does not give rise to risks of derailment.
Railway operators in the AAR area are substantially divided between passenger and goods operators. Accordingly, there is a clear separation between railway networks for passenger traffic and those for goods transport, and there is no need to provide uniform operating requirements for the two types of traffic.
In the AAR area, the power of the radio signal between the master system MS and the slave systems SS has values close to 30 W.
Conversely, in the European environment (the UIC area), passenger traffic has to share the rail network with goods traffic, and both types of train have to obey the same regulations and comply with the same stopping distances.
The maximum radio transmission power permitted by the ETSI (European Telecommunication Standards Institute) standards at frequencies around 450 MHz is currently 500 mW, and it cannot be assumed that authorization will be obtained in future to increase the transmission power to values above 1 W for railway applications, although even this level is much lower than the power permitted by AAR regulations.
Furthermore, European rail networks have more tunnels than American goods traffic networks. It is known from the theory of propagation that frequencies of around 450 MHz undergo high attenuation in tunnels, and in a situation where the transmitting system and the receiving system are outside the opposite ends of a tunnel, communication may be blocked entirely. The probability of losing radio communication in UIC applications is therefore considerably higher than in AAR applications.
The above description makes it clear that a distributed power system according to AAR specifications, if operated according to European standards and conditions, provides a substantially reduced level of availability, and entails a higher risk of obstruction to passenger traffic. Furthermore, the Comm Loss Idle Down procedure as defined for the AAR area cannot be applied to a UIC train with a length of more than 750 m.
U.S. Pat. No. 8,190,311 B2 describes a solution, partly anticipated in EP 0 983 920, which can be implemented, for example, via the Locotrol (registered trademark) system, such that, by contrast with the provisions of the Comm Loss Idle Down procedure, if radio communication is absent, and if there is a reduction of pressure in the brake pipe BP generated by the master system MS and detected by the slave systems SS on board the slave locomotives SL, where this pressure reduction is greater than a predetermined value, for example 0.5 bar (about 7 psi), the slave systems SS on board the slave locomotives SL apply, for their part, a permanent pressure reduction of 0.7 bar (about 12 psi) or more, contributing to the further evacuation of the brake pipe BP at the points along the train where the locomotive SL are positioned, thus shortening braking times and, above all, limiting the longitudinal stresses. The removal of the pressure reduction applied by the slave locomotives SL takes place only when radio communication is restored.
However, this solution only partially resolves the problems of the applicability of distributed power methods as used in the AAR area. This is because, although the procedure according to U.S. Pat. No. 8,190,311 B2 partially reduces the longitudinal stresses and the risks of derailment, it does not provide a response to the need to continue to manoeuvre the train, even in degraded mode, if the absence of radio communication persists.